Update notif corner roundness on drag and snapback

If a dragged view is the first or last view in its section,
remove it from views considered for roundness updates so that
the dragged view becomes square and bordering notif corners become
round.

Bug: 171817112
Test: half swipe first notif in section => 2nd notif rounds
Test: half swipe last notif in section => 2nd to last notif rounds
Test: half swipe middle notif in section => no change to roundness
Test: half swipe then let go to snap back => corners restored
Test: full swipe to dismiss => new roundness preserved

Change-Id: I404e45f2548ab2f0fa77a9fc0457aaf864a4fe88